REGULATED UNDEREXPRESSION OF THE FLIM PROTEIN OF ESCHERICHIA-COLI ANDEVIDENCE FOR A LOCATION IN THE FLAGELLAR MOTOR DISTINCT FROM THE MOTAMOTB TORQUE GENERATORS/

The FliM protein of Escherichia coli is essential for the assembly and function of flagella. Here, we report the effects of controlled low-l evel expression of FliM in a fliM null strain, Disruption of the fliM gene abolishes flagellation. Underexpression of FliM causes cells to p roduce comparatively few flagella, add most flagella built are defecti ve, producing subnormal average torque and fluctuating rapidly in spee d, The results imply that in a normal flagellar motor, multiple molecu les of FliM are present and can function independently to some degree. The speed fluctuations indicate that stable operation requires most, possibly all, of the normal complement of FIM, Thus, the FliM subunits are not as fully independent as the motility proteins MotA and MotB c haracterized in earlier work, suggesting that FliM occupies a location in the motor distinct from the MotA/MotB torque generators. Several m utations in fliM previously reported to cause flagellar paralysis in S almonella typhimurium (H, Sockett, S, Yamaguchi, M, Kihara, V, M, Irik ura, and R, M, Macnab, J. Bacteriol, 174:793-806, 1992) were made and characterized in E. coli, These mutations did not cause flagellar para lysis in E. coli; their phenotypes were more complex and suggest that FliM is not directly involved in torque generation.